If a mixed colour dries darker than expected what adjustment should be made next time?

Correct answer: Prepare a slightly lighter mixture while wet

The governing concept is the drying shift: many paint mixtures appear slightly darker or less bright after water or medium evaporates. Therefore, the artist should prepare the wet mixture a little lighter than the intended final colour and check a small test swatch before applying it widely. Making it darker or adding black would worsen the problem, while avoiding mixing does not correct the value.